If it is less important - like VP and no base - use GAR+ART. That is usually enough against AI, more so if the terrain favors defence (mountain, jungle)
If there is flat land and/or you need to protect a base use 3 INF plus either ART/ART/ENG or ART/AA/ENG. Always put in the Pioneers because that way all DIVs that are in the same army dig in not only 20 but 40 and have more defence value. That way you are less dependend on costal forts, and you save IC. Dont count on the forts too much, they are good but can be bombed and they tend to get destroyed in a long combat (loosing their benifits until they are rebuild) - the 40 entrenchment bonus stays no matter what.
Cheaper but still good is INF/INF/MIL with ART/ENG for the INF. Lots of advantages and nearly the same effect. Less supply consumption, less IC, less MP and so on. When fully dug in no bombing can harm you anyway so AA is not so important.
Also you can consider using GAR on islands where there is no retreat possible anyway - and using INF in places where you can retreat (if there is no fleet blocking you that is) Switch on victory or death in any case, just to be save.